source: branches/minix3-book/servers/pm/forkexit.c@ 4

/* This file deals with creating processes (via FORK) and deleting them (via
 * EXIT/WAIT).  When a process forks, a new slot in the 'mproc' table is
 * allocated for it, and a copy of the parent's core image is made for the
 * child.  Then the kernel and file system are informed.  A process is removed
 * from the 'mproc' table when two events have occurred: (1) it has exited or
 * been killed by a signal, and (2) the parent has done a WAIT.  If the process
 * exits first, it continues to occupy a slot until the parent does a WAIT.
 *
 * The entry points into this file are:
 *   do_fork:    perform the FORK system call
 *   do_pm_exit: perform the EXIT system call (by calling pm_exit())
 *   pm_exit:    actually do the exiting
 *   do_wait:    perform the WAITPID or WAIT system call
 */

#include "pm.h"
#include <sys/wait.h>
#include <minix/callnr.h>
#include <minix/com.h>
#include <signal.h>
#include "mproc.h"
#include "param.h"

#define LAST_FEW            2   /* last few slots reserved for superuser */

FORWARD _PROTOTYPE (void cleanup, (register struct mproc *child) );

/*===========================================================================*
 *                              do_fork                                      *
 *===========================================================================*/
PUBLIC int do_fork()
{
/* The process pointed to by 'mp' has forked.  Create a child process. */
  register struct mproc *rmp;   /* pointer to parent */
  register struct mproc *rmc;   /* pointer to child */
  int child_nr, s;
  phys_clicks prog_clicks, child_base;
  phys_bytes prog_bytes, parent_abs, child_abs; /* Intel only */
  pid_t new_pid;

 /* If tables might fill up during FORK, don't even start since recovery half
  * way through is such a nuisance.
  */
  rmp = mp;
  if ((procs_in_use == NR_PROCS) || 
                (procs_in_use >= NR_PROCS-LAST_FEW && rmp->mp_effuid != 0))
  {
        printf("PM: warning, process table is full!\n");
        return(EAGAIN);
  }

  /* Determine how much memory to allocate.  Only the data and stack need to
   * be copied, because the text segment is either shared or of zero length.
   */
  prog_clicks = (phys_clicks) rmp->mp_seg[S].mem_len;
  prog_clicks += (rmp->mp_seg[S].mem_vir - rmp->mp_seg[D].mem_vir);
  prog_bytes = (phys_bytes) prog_clicks << CLICK_SHIFT;
  if ( (child_base = alloc_mem(prog_clicks)) == NO_MEM) return(ENOMEM);

  /* Create a copy of the parent's core image for the child. */
  child_abs = (phys_bytes) child_base << CLICK_SHIFT;
  parent_abs = (phys_bytes) rmp->mp_seg[D].mem_phys << CLICK_SHIFT;
  s = sys_abscopy(parent_abs, child_abs, prog_bytes);
  if (s < 0) panic(__FILE__,"do_fork can't copy", s);

  /* Find a slot in 'mproc' for the child process.  A slot must exist. */
  for (rmc = &mproc[0]; rmc < &mproc[NR_PROCS]; rmc++)
        if ( (rmc->mp_flags & IN_USE) == 0) break;

  /* Set up the child and its memory map; copy its 'mproc' slot from parent. */
  child_nr = (int)(rmc - mproc);        /* slot number of the child */
  procs_in_use++;
  *rmc = *rmp;                  /* copy parent's process slot to child's */
  rmc->mp_parent = who;                 /* record child's parent */
  /* inherit only these flags */
  rmc->mp_flags &= (IN_USE|SEPARATE|PRIV_PROC|DONT_SWAP);
  rmc->mp_child_utime = 0;              /* reset administration */
  rmc->mp_child_stime = 0;              /* reset administration */

  /* A separate I&D child keeps the parents text segment.  The data and stack
   * segments must refer to the new copy.
   */
  if (!(rmc->mp_flags & SEPARATE)) rmc->mp_seg[T].mem_phys = child_base;
  rmc->mp_seg[D].mem_phys = child_base;
  rmc->mp_seg[S].mem_phys = rmc->mp_seg[D].mem_phys + 
                        (rmp->mp_seg[S].mem_vir - rmp->mp_seg[D].mem_vir);
  rmc->mp_exitstatus = 0;
  rmc->mp_sigstatus = 0;

  /* Find a free pid for the child and put it in the table. */
  new_pid = get_free_pid();
  rmc->mp_pid = new_pid;        /* assign pid to child */

  /* Tell kernel and file system about the (now successful) FORK. */
  sys_fork(who, child_nr);
  tell_fs(FORK, who, child_nr, rmc->mp_pid);

  /* Report child's memory map to kernel. */
  sys_newmap(child_nr, rmc->mp_seg);

  /* Reply to child to wake it up. */
  setreply(child_nr, 0);                /* only parent gets details */
  rmp->mp_reply.procnr = child_nr;      /* child's process number */
  return(new_pid);                      /* child's pid */
}

/*===========================================================================*
 *                              do_pm_exit                                   *
 *===========================================================================*/
PUBLIC int do_pm_exit()
{
/* Perform the exit(status) system call. The real work is done by pm_exit(),
 * which is also called when a process is killed by a signal.
 */
  pm_exit(mp, m_in.status);
  return(SUSPEND);              /* can't communicate from beyond the grave */
}

/*===========================================================================*
 *                              pm_exit                                      *
 *===========================================================================*/
PUBLIC void pm_exit(rmp, exit_status)
register struct mproc *rmp;     /* pointer to the process to be terminated */
int exit_status;                /* the process' exit status (for parent) */
{
/* A process is done.  Release most of the process' possessions.  If its
 * parent is waiting, release the rest, else keep the process slot and
 * become a zombie.
 */
  register int proc_nr;
  int parent_waiting, right_child;
  pid_t pidarg, procgrp;
  struct mproc *p_mp;
  clock_t t[5];

  proc_nr = (int) (rmp - mproc);        /* get process slot number */

  /* Remember a session leader's process group. */
  procgrp = (rmp->mp_pid == mp->mp_procgrp) ? mp->mp_procgrp : 0;

  /* If the exited process has a timer pending, kill it. */
  if (rmp->mp_flags & ALARM_ON) set_alarm(proc_nr, (unsigned) 0);

  /* Do accounting: fetch usage times and accumulate at parent. */
  sys_times(proc_nr, t);
  p_mp = &mproc[rmp->mp_parent];                        /* process' parent */
  p_mp->mp_child_utime += t[0] + rmp->mp_child_utime;   /* add user time */
  p_mp->mp_child_stime += t[1] + rmp->mp_child_stime;   /* add system time */

  /* Tell the kernel and FS that the process is no longer runnable. */
  tell_fs(EXIT, proc_nr, 0, 0);  /* file system can free the proc slot */
  sys_exit(proc_nr);

  /* Pending reply messages for the dead process cannot be delivered. */
  rmp->mp_flags &= ~REPLY;
  
  /* Release the memory occupied by the child. */
  if (find_share(rmp, rmp->mp_ino, rmp->mp_dev, rmp->mp_ctime) == NULL) {
        /* No other process shares the text segment, so free it. */
        free_mem(rmp->mp_seg[T].mem_phys, rmp->mp_seg[T].mem_len);
  }
  /* Free the data and stack segments. */
  free_mem(rmp->mp_seg[D].mem_phys,
      rmp->mp_seg[S].mem_vir 
        + rmp->mp_seg[S].mem_len - rmp->mp_seg[D].mem_vir);

  /* The process slot can only be freed if the parent has done a WAIT. */
  rmp->mp_exitstatus = (char) exit_status;

  pidarg = p_mp->mp_wpid;               /* who's being waited for? */
  parent_waiting = p_mp->mp_flags & WAITING;
  right_child =                         /* child meets one of the 3 tests? */
        (pidarg == -1 || pidarg == rmp->mp_pid || -pidarg == rmp->mp_procgrp);

  if (parent_waiting && right_child) {
        cleanup(rmp);                   /* tell parent and release child slot */
  } else {
        rmp->mp_flags = IN_USE|ZOMBIE;  /* parent not waiting, zombify child */
        sig_proc(p_mp, SIGCHLD);        /* send parent a "child died" signal */
  }

  /* If the process has children, disinherit them.  INIT is the new parent. */
  for (rmp = &mproc[0]; rmp < &mproc[NR_PROCS]; rmp++) {
        if (rmp->mp_flags & IN_USE && rmp->mp_parent == proc_nr) {
                /* 'rmp' now points to a child to be disinherited. */
                rmp->mp_parent = INIT_PROC_NR;
                parent_waiting = mproc[INIT_PROC_NR].mp_flags & WAITING;
                if (parent_waiting && (rmp->mp_flags & ZOMBIE)) cleanup(rmp);
        }
  }

  /* Send a hangup to the process' process group if it was a session leader. */
  if (procgrp != 0) check_sig(-procgrp, SIGHUP);
}

/*===========================================================================*
 *                              do_waitpid                                   *
 *===========================================================================*/
PUBLIC int do_waitpid()
{
/* A process wants to wait for a child to terminate. If a child is already 
 * waiting, go clean it up and let this WAIT call terminate.  Otherwise, 
 * really wait. 
 * A process calling WAIT never gets a reply in the usual way at the end
 * of the main loop (unless WNOHANG is set or no qualifying child exists).
 * If a child has already exited, the routine cleanup() sends the reply
 * to awaken the caller.
 * Both WAIT and WAITPID are handled by this code.
 */
  register struct mproc *rp;
  int pidarg, options, children;

  /* Set internal variables, depending on whether this is WAIT or WAITPID. */
  pidarg  = (call_nr == WAIT ? -1 : m_in.pid);     /* 1st param of waitpid */
  options = (call_nr == WAIT ?  0 : m_in.sig_nr);  /* 3rd param of waitpid */
  if (pidarg == 0) pidarg = -mp->mp_procgrp;    /* pidarg < 0 ==> proc grp */

  /* Is there a child waiting to be collected? At this point, pidarg != 0:
   *    pidarg  >  0 means pidarg is pid of a specific process to wait for
   *    pidarg == -1 means wait for any child
   *    pidarg  < -1 means wait for any child whose process group = -pidarg
   */
  children = 0;
  for (rp = &mproc[0]; rp < &mproc[NR_PROCS]; rp++) {
        if ( (rp->mp_flags & IN_USE) && rp->mp_parent == who) {
                /* The value of pidarg determines which children qualify. */
                if (pidarg  > 0 && pidarg != rp->mp_pid) continue;
                if (pidarg < -1 && -pidarg != rp->mp_procgrp) continue;

                children++;             /* this child is acceptable */
                if (rp->mp_flags & ZOMBIE) {
                        /* This child meets the pid test and has exited. */
                        cleanup(rp);    /* this child has already exited */
                        return(SUSPEND);
                }
                if ((rp->mp_flags & STOPPED) && rp->mp_sigstatus) {
                        /* This child meets the pid test and is being traced.*/
                        mp->mp_reply.reply_res2 = 0177|(rp->mp_sigstatus << 8);
                        rp->mp_sigstatus = 0;
                        return(rp->mp_pid);
                }
        }
  }

  /* No qualifying child has exited.  Wait for one, unless none exists. */
  if (children > 0) {
        /* At least 1 child meets the pid test exists, but has not exited. */
        if (options & WNOHANG) return(0);    /* parent does not want to wait */
        mp->mp_flags |= WAITING;             /* parent wants to wait */
        mp->mp_wpid = (pid_t) pidarg;        /* save pid for later */
        return(SUSPEND);                     /* do not reply, let it wait */
  } else {
        /* No child even meets the pid test.  Return error immediately. */
        return(ECHILD);                      /* no - parent has no children */
  }
}

/*===========================================================================*
 *                              cleanup                                      *
 *===========================================================================*/
PRIVATE void cleanup(child)
register struct mproc *child;   /* tells which process is exiting */
{
/* Finish off the exit of a process.  The process has exited or been killed
 * by a signal, and its parent is waiting.
 */
  struct mproc *parent = &mproc[child->mp_parent];
  int exitstatus;

  /* Wake up the parent by sending the reply message. */
  exitstatus = (child->mp_exitstatus << 8) | (child->mp_sigstatus & 0377);
  parent->mp_reply.reply_res2 = exitstatus;
  setreply(child->mp_parent, child->mp_pid);
  parent->mp_flags &= ~WAITING;         /* parent no longer waiting */

  /* Release the process table entry and reinitialize some field. */
  child->mp_pid = 0;
  child->mp_flags = 0;
  child->mp_child_utime = 0;
  child->mp_child_stime = 0;
  procs_in_use--;
}